shellcheck: enable shell linting

Add to the `format-code.sh` optional support for linting shell scripts
using `shellcheck`.  All repositories will have this ran, but only
repositories with a `.shellcheck` file in the root of the repository
will stop on error.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ie25c27cffc96b7fa66291abef1b0f90a0f1862ed
2 files changed